PROJECTSUMMARY: Organs and other complex tissues ?know? when and how to stop growing to arrive at the correct size and shape. Disruption of organ size control mechanisms can lead to congenital abnormalities, poor organ homeostasis and tissue repair, and tumors. Adult zebrafish caudal fins, including their complex skeleton and other tissues, perfectly regenerate to their original size and shape regardless of the nature or position of the injury. Therefore, zebrafish fin regeneration provides a compelling and genetically tractable vertebrate model tointerrogateorgansizecontrolmechanisms.Prevailingmodelsforrobustfinsizeregenerationspeculatethat fin cells maintain a multitude of ?positional identities? that somehow instruct different degrees of outgrowth. We propose a distinctandstraightforwardmodel that neatly explains how fin size and shape is restored without invoking molecularly encoded positional information. A key cell population at the distal end of the regenerating fin that we term the ?niche? produces Wnt signals that promote fin outgrowth by sustaining progenitor cells. We identify Dachsund transcription factors as novel niche markers and show that the niche uniquely forms from intra-??ray mesenchyme that populates the inside of the cylindrical, differentially sized, and progressively tapered fin rays. We show that the niche, and therefore Wnt, steadily dissipates as regenerationunfolds;onceexhausted,growthstops.Assuch,regeneratedfinsizeisdictatedbytheamountof niche formed upon damage ? which is simply dependent on the availability of intra-??ray mesenchyme and hence bone width at the damage site. This ?transpositional scaling? model suggests that macro-??scale fin size and shape is determined by mesenchyme-??niche state transitions and self-??restoring bone geometry rather than uniquepositionalidentitiesofindividualcells.Wewillexplorethisparadigmanduncoverunderlyingcelland molecular mechanisms for size control during fin regeneration by three Specific Aims: 1. Define intra-??ray mesenchyme/distalnichelineagecellstates,transitions,andfates,2.Determinesignalingandtranscriptional mechanisms maintaining niche state and function, and 3. Determine niche ?countdown timer? mechanisms usinglongfint2zebrafish?whichweshowhaveabrokentimerduetomisexpressionofthekcnh2aionchannel. ThisinsightsuggestsionchannelsandCa2+signalinggovernnichecellself-??renewal.Ourprogramwillsupport a potentially broadly applicable ?transpositional scaling? concept with exemplary mechanisms for how organ size and shape are determined by dynamic populations of tissue-??resident niche cells. Our study will have additional human health impacts since 1) understanding bone regeneration in zebrafish may inform regenerativemedicineapproachesforhumanbonedisease,and2)kcnh2aisthezebrafishorthologueofkcnh2, which is commonly mutated in long QT syndrome and encodes a protein that is a notorious therapeutic ?off-?? target?.Ourparadigmaticanddiversetechnologicalinnovationswillopenupnewdirectionsandinspireother scientists,broadeningourproject?simpactonbothfundamentalresearchandregenerativemedicine.